The proposed research is designed to study the expression of H-2 linked loci in functionally different lymphocyte subpopulations. Particular emphasis will be given to identifying Ia loci which control cell surface determinants selectively expressed in a given subclass. The possible role products of these loci play in the function mediated by immunocompetent cells will also be examined. Restricted antisera will be prepared against the products of separate loci. Tests for expression of each locus in different functional lymphocyte subpopulations will be conducted in negative selection experiments utilizing antiserum plus complement. Expression in nonimmune spleen cells (virgin B lymphocytes and unprimed helper T lymphocytes) will be examined in in vitro Mishell-Dutton cultures. Expression in hapten-primed (memory B) and carrier-primed (helper T) lymphocytes will be determined in in vivo secondary adoptive transfers and in in vitro Mishell-Dutton cultures. Expression in suppressor T lymphocytes (allotype (Ig-1b) and carrier (KLH specific) will be evaluated in in vivo adoptive transfers. Antisera killing a given lymphocyte subclass will be absorbed with fractionated cell populations (B lymphocytes, Ly-1 and Ly-2 bearing T lymphocytes) from normal, primed, and suppressed animals in order to further assess the extent of selective expression of the loci, and the stage of cell maturation at which the loci are expressed. Absorptions will also be conducted with thymus, macrophages, bone marrow and epithelial cells. In addition, the presence of distinct products in the membrane of each cell type will be evaluated by indirect immunofluorescence. The role different cell surface determinants play in the immune response will be examined by attempting to block cellular interactions with restricted antisera in vivo and in vitro.